When Combustible Gearhulk enters the battlefield, target opponent may have you draw three cards. If the player doesn't, put the top three cards of your library into your graveyard, then Combustible Gearhulk deals damage to that player equal to the total converted mana cost of those cards.

If any of the cards put into your graveyard have in their mana cost, X is considered to be 0.

9/20/2016

If you have fewer than three cards in your library, the opponent may choose to have you draw three cards. If so, you’ll lose the game as state-based actions are performed.

9/20/2016

While Combustible Gearhulk’s triggered ability is resolving, no player may take any actions other than those specified by the Gearhulk. Once the opponent makes a choice, players can’t cast spells or activate abilities until after the cards are drawn or put into the graveyard and damage has been dealt (if applicable).